Traditionally, lawn and garden power tools are powered by a gasoline engine, 120-volt alternating current (AC) power source, or a direct current (DC) battery power source. Woodworking, metalworking, and construction power tools are also powered by a gasoline engine, 120-volt (V) AC power source, or a DC battery power source.
Connecting multiple battery packs together can increase the capacity of the battery system. For example, connecting multiple battery packs in parallel generally increases the capacity (ampere-hours) of the battery system, while the combined output (e.g., 18V) retains the voltage of the individual battery packs (e.g., 18V). However, if the voltages of the battery packs, connected in parallel are not approximately equal, issues can arise. The term battery pack and battery as used herein may be used interchangeably to refer to a battery pack.
One such issue for parallel-connected battery packs is cross-charging. Cross-charging can occur if one battery pack is at a reasonably higher voltage, or state of charge, than the voltage of another parallel-connected battery pack. If this occurs, current from the battery pack at the higher voltage will flow into the battery pack at the lower voltage. This current can become relatively large (e.g., 25 ampere (A)) and, therefore, can be hazardous to the battery packs, especially in certain operating conditions (e.g., at low temperature). Cross-charging can also occur when the battery packs connected in parallel have unequal capacity size.
Another issue for parallel-connected battery packs may arise if one of the battery packs connected in parallel becomes fully discharged before the other battery pack. If this occurs, the battery pack that is fully discharged, but still connected, can be permanently damaged.
Another issue for parallel connected battery packs may arise if one of the battery packs connected in parallel becomes over-heated. If this occurs, the battery pack which is over-heated, but still connected, can be permanently damaged.
Series connected battery packs may also have similar issues, as described in more detail below.
Embodiments of the invention are operable to overcome such issues, as well as other issues related to a power tool operable to receive and be powered by multiple battery packs.